Further characterization of the endogenous C-type viruses of mice, especially the radiation leukemia virus (RadLV), a leukemogenic endogenous virus extractable from X-ray-induced thymic lymphomas of strain C57BL/Ka mice; studies of the factors which determine the inducibility, kinetics of replication, and host range of these viruses in vitro; and studies of the complex interactions among (1) external physical or chemical leukemogenic agents, (2) endogenous viruses, (3) the lymphoid cell populations which are the target cells for neoplastic transformation, and (4) the internal host environment, with special reference to the thymus and the immune system.